udp: Make udp[46]_seq_show() static.

udp: Make udp[46]_seq_show() static.

Since commit a3d2599b2446 ("ipv{4,6}/udp{,lite}: simplify proc
registration"), udp4_seq_show() and udp6_seq_show() are not
used in net/ipv4/udplite.c and net/ipv6/udplite.c.

Instead, udp_seq_ops and udp6_seq_ops are exposed to UDP-Lite.

Let's make udp4_seq_show() and udp6_seq_show() static.

udp_seq_ops and udp6_seq_ops are moved to udp_impl.h so that
we can make them static when the header is removed.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260311052020.1213705-2-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>